Method of manufacturing membranes and the resulting membranes

ABSTRACT

This invention provides a process for making microporous membranes from a polymer solution and the membranes therefrom. A thermal assist, such as heating of the polymer solution can be effected subsequent to shaping the solution, such as by forming a film, tube or hollow fiber of the solution under conditions that do not cause phase separation.. In a preferred embodiment, the formed solution is briefly heated to generate a temperature gradient through the body of the formed solution. The polymer in solution then is precipitated to form a microporous structure. The formation of a wide variety of symmetric and asymmetric structures can be obtained using this process. Higher temperatures and/or longer heating times effected during the heating step result in larger pore sizes and different pore gradients in the final membrane product.

FIELD OF THE INVENTION

[0001] This invention relates to a process for making asymmetricmicroporous membranes having a controlled pore size and architecture andto the membranes so produced. More particularly, this invention relatesto a process for making these microporous membranes from a polymersolution that is selectively heated under controlled conditions toinduce a predetermined membrane architecture formed in a subsequentphase separation step.

BACKGROUND OF THE INVENTION

[0002] Microporous membranes based on semi-crystalline polymers havebeen previously prepared. Most of the commercial membranes of thesepolymers are symmetric in nature. The production of such microporousmembranes are described, for example, in U.S. Pat. No. 4,208,848 forPVDF and in U.S. Pat. No. 4,340,479 for polyamide membranes. Thesepreparations are generally described to consist of the following steps:a) preparation of a specific and well controlled polymer solution, b)casting the polymer solution onto a temporary substrate, c) immersingand coagulating the resulting film of the polymer solution in anonsolvent, d) removing the temporary substrate and e) drying theresulting microporous membrane.

[0003] Polyvinylidene fluoride (PVDF) membranes as described above aremade by casting a lacquer in a specific coagulant (e.g. acetone-watermixture, IPA-water mixture or methanol) that allows the formation of amicroporous, symmetric membrane. A similar process is used for symmetricpolyamide membranes. In these prior art processes, the semi-crystallinepolymers used primarily lead to symmetric membranes. Membranes made fromsuch semi-crystalline polymers have a characteristic property wherebythe thermal history of the polymer solution prior to casting has adramatic effect on membrane performance. In general terms, it has beenfound that the higher the maximum temperature to which the solution isheated to, the larger the rated pore size of the resulting microporousmembrane. In one method of controlling pore size, the polymer solutionis made at a relatively low temperature in a typical manufacturingstirred tank vessel, or similar, and then heated to the desired maximumtemperature by, for example a heated jacket. Variability in lacquerhistory can therefore cause reduced process yields. It can beappreciated that fine control over the thermal history of a large massof viscous solution is difficult. In-line heating and cooling treatmentis sometimes used in order to provide improved control over the thermalhistory of the polymer solution being processed. An in-line processprovides a means for heating the solution as it is transported through apipeline, thereby reducing the effective mass of solution being heated.The shorter heating contact time necessitated by in-line heatingrequires good mixing to obtain even heat treatment. Membranes made fromsolutions having a uniform thermal history throughout its bulk tend toproduce symmetric membranes.

[0004] Microporous membranes are described as symmetric or asymmetric.Symmetric membranes have a porous structure with a pore sizedistribution characterized by an average pore size that is substantiallythe same through the membrane. In asymmetric membranes, the average poresize varies through the membrane, in general, increasing in size fromone surface to the other. Other types of asymmetry are known. Forexample, those in which the pore size goes through a minimum pore sizeat a position within the thickness of the membrane. Asymmetric membranestend to have higher fluxes compared to symmetric membranes of the samerated pore size and thickness. Also, it is well known that asymmetricmembranes can be used with the larger pore side facing the fluid streambeing filtered, creating a prefiltration effect.

[0005] Practitioners have developed complex methods to produceasymmetric membranes from semi-crystalline polymers. PVDF membranes areproduced by thermally induced phase separation (TIPS), where thetemperature of an extruded film, tube or hollow fiber of a homogeneouspolymer solution is quenched down to a lower temperature therebyinducing phase separation. Examples of PVDF membranes made by TIPS aredisclosed in U.S. Pat. Nos. 4,666,607, 5,013,339 and 5,489,406. Theseprocesses require high temperatures and screw type extruders, increasingprocess complexity.

[0006] U.S. Pat. No. 4,629,563 to Wrasidlo discloses asymmetricmembranes that can be characterized by a skinned layer that isrelatively dense and thick with a gradually changing pore size beneaththe skinned layer. Claimed ratios of pore sizes in opposite surfacesranges from 10 to 20,000 times. This process requires the use of an“unstable liquid dispersion.” Use of such dispersions reduces thecontrol available over the overall process.

[0007] U.S. Pat. Nos. 4,933,081 and 5,834,107 disclose humid airexposure applied to PVDF-PVP solutions to create PVDF membranes toproduce microporous membranes with high flux characteristics. By usingsimilar humid air exposure techniques as in U.S. Pat. No. 4,629,563,some subtle but apparently important differentiations are made from theWrasidlo patent. These patents teach that differences in lacquercomposition and humid air exposures can lead to large structuralchanges. In U.S. Pat. No. 4,933,081, membranes having hourglass porousstructure are produced with the average diameters of the poresdecreasing along a line from a microporous surface to a coarse poresurface. Thereafter, the pore size increases again along that same line.Both methods require additional control of the humid atmosphere-polymersolution contact time, humid air velocity, temperature and humidity,thereby increasing process complexity.

[0008] Furthermore, U.S. Pat. No. 5,834,107 describes structures havinga gradual changing pore size from microporous side to a coarse surface.All the structures also have some large open volumes in portions of themembrane near the coarse surface of the membrane. This structure isdefined in the patent as filamentous webs. The large open volumes,although they may be different in origin from macrovoids, can causesimilar mechanical failures in membrane application and are thereforenot desirable in applications where high integrity is required. Thepresence of these large open volumes is not beneficial in terms ofretention, since the diameter of the flow paths are much too large toretain typical solutes or particles filtered by microporous membranes.In addition, the methods described above always use a high molecularweight additive in the lacquer and humid air exposure.

[0009] U.S. Pat. No. 6,013,688 discloses making PVDF membranes thatappear to have an isotropic structure, containing a dense array ofclosely aligned and contiguous polymer particles. A part of thestructure is characterized by so-called spherical craters. Suchstructures tend to be mechanically weak.

[0010] U.S. Pat. Nos. 5,626,805 and 5,514,461 disclose a complexthermally induced phase separation technique (TIPS) that quenches bothsides of a film of a polymer solution with a different rate to effectdifferent supersaturation in different time frames. The thermal quenchtechnique can lead to asymmetric structures being characterized in crosssection by a beady, open structure at one surface and a leafy, moretight structure at the other surface. However, to obtain an improvementin flux, it is not sufficient to have a larger pore size on bothsurfaces, but also that the pore size changes throughout the membrane.

[0011] U.S. Pat. No. 5,444,097 discloses heat induced phase separationfor making high flow membranes. This method depends on generating phaseseparation by heating of a polymer solution to above its lower criticalsolution temperature (LCST). The LCST is a temperature at which apolymer solution becomes cloudy due to phase separation of the solution.A minimum on a curve of cloud point temperature as a function of polymerconcentration is referred to the lower critical solution temperature.This technique is very specific for polymer solutions that arecharacterized by a lower critical solution temperature (LCST). In thisprocess the polymer solution must be maintained at the desiredtemperature above the LCST. This increases the complexity of the processbecause the solvent laden solution must be transported from the heatingregion to the immersion region of the process while maintaining thedesired temperature in a narrow temperature zone above or below thedesired temperature so as not to change the final pore size from thedesigned pore size.

[0012] Accordingly, it would be desirable to provide a simple, easilycontrolled process for forming microporous membranes having asymmetricpore structure wherein the pore size throughout the membrane thicknessvaries.

SUMMARY OF THE INVENTION

[0013] This invention provides a process for making microporousmembranes from a polymer solution. While it is well-known that themaximum temperature certain solutions of semi-crystalline polymers reachcontrols the pore size of the resulting membrane, it has beensurprisingly found in this invention that by a brief thermal assist suchas heating, much shorter in duration than taught in the prior art, ofthe formed polymer solution, so as to produce a temperature gradientthrough at least a portion of the thickness of the formed solution,produces a membrane having a controlled degree of asymmetry and poresize. Symmetric membranes having controlled pore size can be produced bya thermal assist such as heating the formed solution to a uniformtemperature through the thickness of the formed solution.

[0014] A thermal assist is an application of heat across the thicknessof a formed solution. A thermal assist can be accomplished by heating asurface of a formed solution or by a combination of heating one surfaceand previously, simultaneously or subsequently cooling the other side ofthe formed solution. Also by cooling one side and heating of the otherside, a thermal assist can be accomplished.

[0015] In the process, the polymer solution is thermally assisted underconditions that prevent phase separation. Thermal assisting of thepolymer solution can be effected subsequent to shaping the solution,such as by forming a film, tube or hollow fiber of the solution. Thisinvention will be described for convenience in terms of a film or sheetmembranes, without being limited thereby. In a preferred embodiment, theformed solution is briefly thermally assisted to generate a temperatureprofile through the body of the formed solution. The polymer in solutionthen is precipitated to form a microporous structure such as by beingimmersed in a bath of nonsolvent for the polymer or by evaporation ofsolvent, either of these steps optionally in conjunction with contactwith humid air before or during phase separation. Higher temperaturesand/or longer times effected during the thermal assist step result inlarger pore sizes and different profiles in the final membrane product.

[0016] In one preferred embodiment, the thermal assist step produces atemperature gradient in the formed polymer solution film that results inan asymmetric membrane being formed.

[0017] The preferred thermal assistance is by heating. Heating can alsobe done to produce a uniform temperature gradient through the body ofthe formed polymer solution film, so that a symmetric membrane can beformed in the subsequent phase separation step.

[0018] The length of time an element of volume in the body of a formedpolymer solution film remains at the highest temperature also affectsfinal structure. This invention therefore discloses control of totalthermal assistance time and the thermal gradient formed in at least aportion of the formed polymer solution film.

[0019] The microporous product produced by the process of this inventioncan be skinned or unskinned and can be symmetric or asymmetric. Themicroporous structures produced by the process of the invention are freeof macrovoids that are substantially larger than the average pore sizeof the membrane. The term “macrovoids” as used herein refers to voids ina membrane that are sufficiently large as not to function to produce aretentate. Additionally, the structures of the present invention arefree of the filamentous webs of the prior art that also causeinefficient filtration.

[0020] Additionally, one is able to form composite structures of two ormore layers wherein at least of the layers is formed by the thermalassist method of the present invention.

[0021] Furthermore, this thermal assist can be used to create symmetricmembranes with variable pore size depending on the process condition.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a side view of an apparatus useful in effecting theprocess of this invention.

[0023]FIG. 2 is a graph of bubble point of the membranes of thisinvention as a function of air flux.

[0024]FIG. 3 is a photomicrograph of a cross-section of an asymmetricmicroporous membrane of this invention.

[0025]FIG. 4 is a photomicrograph of the top surface of the membrane ofFIG. 3.

[0026]FIG. 5 is a photomicrograph of the bottom surface of the membraneof FIG. 3.

DESCRIPTION OF SPECIFIC EMBODIMENTS

[0027] This invention comprises a process for making porous polymericstructures from formed polymer solutions wherein the thermal profile ofthe formed solution is controlled. The inventor has found that byproducing a controlled temperature gradient in a formed solution of asemi-crystalline polymer prior to a phase separation step, one is ableto produce porous structures having structural characteristics withcontrolled pore size gradients. It is essential to effect polymerprecipitation subsequent to a thermal step and to avoid precipitationprior to and during the thermal step.

[0028] The process of this invention permits the formation of membraneshaving varying asymmetries. Asymmetry refers to the variation of averagepore size in the thickness direction of a porous structure. For a sheet,asymmetry refers to the change in average pore size traversing thethickness from one side of the sheet to the opposite side. For a hollowfiber membrane, asymmetry refers to the change in average pore sizetraversing the thickness from the inner diameter to the outer diameteror vice versa. Asymmetry can be monotonic, that is, constantlyincreasing through the thickness. Asymmetry can also have an hourglassprofile, in which the average pore size decreases to a minimum and thenincreases through the thickness. Another asymmetry resembles a funnel,having a symmetric portion adjoined to a monotonically increasingasymmetric portion.

[0029] The process of the present invention comprises,

[0030] Preparing a solution of one or more polymers,

[0031] Forming the solution into a shaped object,

[0032] Providing a thermal assist to the formed solution to produce atemperature profile in at least a portion of the thickness of the formedsolution,

[0033] Producing a porous structure by a phase separation process step.

[0034] The polymer solution consists of at least one polymer and atleast one solvent for the polymer or polymers. The solution may containone or more components that are poor solvents or non-solvents for thepolymer or polymers. Such components are sometimes called “porogens” inthe art. The homogeneous solution can optionally contain one or morecomponents, which are non-solvents for said polymer. The polymersolution can either be stable in time (good solvent quality) or bemeta-stable in time. This solution also can potentially have a lowercritical solution temperature or an upper critical solution temperature.Example components of such solutions are well known in the art, and itis not necessary to exhaustively list all possible variations. Examplesof polymers useful in the present invention include but are not limitedto polyvinyl chloride, polyvinylidene fluoride, blends of polyvinylidenefluoride such as with polyvinylpyrrolidone, polyvinylidene fluoridecopolymers and various polyamides, such as the various nylons includingNylon 66. Solvents that are used included such examples as dimethylformamide, N,N-dimethylacetamide, N-methyl pyrrolidone, tetramethylurea,acetone, dimethylsulfoxide. A myriad of porogens have been used in theart, including such examples as formamide, various alcohols andpolyhydric compounds, water, various polyethylene glycols, and varioussalts, such as calcium chloride and lithium chloride.

[0035] The polymer solution is made by blending and mixing in a closedvessel, according to known methods, with the precaution that thetemperature be controlled by cooling means to below the temperature tobe applied in the thermal assisting step. The solution can be optionallyfiltered before the solution forming step.

[0036] The resulting homogeneous solution is formed into the desiredshape by techniques such as casting, coating, spinning, extruding, etc.,which are known in the art. Forming the solution is done to obtain thedesired shape of the end product to be produced, which can be in anyform such as block or a cylinder, a flat sheet, a hollow tube, solid orhollow fiber. For example, to produce a sheet, a knife coater, a slotcoater or a LFC coater can be used as is described in “Microfiltrationand Ultrafiltration Principles and Practice” Leos J. Zeman and Andrew L.Zydney; Marcel Dekker (1996). Hollow fibers can be formed using anannular extrusion die.

[0037] In the preferred mode, the formed solution is briefly heated toproduce a temperature gradient in the thickness of the formed solution.For a sheet being carried on a support, such as a web, one side isbriefly heated by contacting the carrier with a heated rod or otherheated object. Heating is done in such a manner that the formed solutiondoes not reach thermal equilibrium, but rather the unheated side doesnot reach the temperature of the heated side. In another mode, infraredheaters can be used to heat one side or the other in such a manner so asto obtain the desired temperature gradient. It can even be contemplatedthat combinations of heating methods can be used to obtain gradientsfrom each surface to give a kind of “hourglass” asymmetry with a regionof minimal pore size within the thickness of the resultant membrane. Inanother mode, if a symmetric structure is desired, the formed solutionwould be heated to thermal equilibrium, in which case no gradient wouldremain.

[0038] The temperature to which at least a portion of the formed polymersolution is heated and the time of heating depends upon the polymer andsolvent/porogen system utilized to make the polymer solution and by thedesired pore size of the membrane resulting from the process. Thepractitioner will have previously determined the relationship betweenthe maximum temperature the solution sees and the final pore size of theresulting membrane. The minimum temperature to which at least one sideof the formed solution can be heated is constrained to be at or abovethe maximum temperature that the solution attained in any previousprocess step. The practitioner will find that there is a temperatureabove which further heating has little effect on increasing pore size.Within these temperature ranges, the practitioner can vary pore size andasymmetry by control of the temperature to which at least one side ofthe formed solution is raised, and the time of the thermal contact.

[0039] Not only the temperature of the treatment, but the time at whichthe solution is at that temperature affect pore sizes. Longer times atany temperature result in a stronger effect, usually larger pores. Alsothe temperature gradient is reduced. Therefore, a surface of the film incontact with a heat source will have a temperature different than theopposite side positioned away from the heat source. A temperaturegradient is effected through the film and the effect of the gradient onthe pore size will depend on the relative steepness of the gradient, andthe time each portion of the film thickness is at a certain temperature.Therefore, by controlling the heat source temperature, the time of heatapplication, formed polymer solution thickness and solution propertiessuch as heat capacity and viscosity, the properties of membranes of thisinvention can be varied.

[0040] Generally the asymmetric membranes of this invention arecharacterized by a higher flux for a given pore size rating of themembrane. This is desirable to shorten filtration processing timesduring some membrane applications. Associated with this, some gradationin pore size can lead to higher capacities of microporous membranes forretaining filtered particles. By slightly changing the processingconditions, membranes of different asymmetries can be made with the samelacquer, having approximately the same pore size, but with differentpermeability characteristics. Thus, structures are provided havingdifferent asymmetries but with a similar pore size in a particularlayer. The process of this invention is capable of providing microporousmembranes having varying asymmetry independent of pore size. Themicroporous symmetric or asymmetric membranes of this invention have anaverage pore size typically in the range of 0.02 to 10 micrometers.

[0041] The inventive process described herein can be envisaged as makinga wide variety of asymmetrical structures. If the thermal assistance isdone to produce a uniform temperature gradient through the thickness ofthe formed solution, then the resulting membrane will have a more orless uniform gradient of pore sizes, with the largest pore sizes at theside to which the thermal assistance was applied, If the thermalassistance is done so that only a portion of the thickness if affected,then that portion will have an asymmetric structure, and the remainderwill be of a symmetric nature. This is s sometimes called a “funnel”structure. Another type of funnel structure may result if a portion ofthe thickness is subjected to the thermal assistance to an approximatelyuniform temperature and the remainder of the thickness attains atemperature gradient. In this case, the adjacent thickness portion tothe side to which heating was applied will be symmetric and theremainder of the membrane will be asymmetric. If both sides of theformed solution were subjected to the thermal assistance so that twogradients extending from each surface were attained, then one couldexpect that both sides would have larger pore sizes than a region in theinterior of the thickness. By varying the relative temperatures of thesides, a variety of structures would be possible. Microporous membranesof this invention have no large macrovoids or filamentous webstructures. It can therefore be seen that in the hands of one ofordinary skill in the art, many possible structures can be formed, notall of which are needed to be exhaustively presented to illustrate theutility of the present invention.

[0042] Referring to FIG. 1, an apparatus 10 useful for effecting anembodiment of the process of this invention comprises a moving belt 12which contacts an outlet of a knife box 14 from which a polymericsolution is dispensed onto the belt 12. The belt 12 supporting a film ofthe polymer solution is passed over heated pipe 16 to effect brieflyheating of the polymer solution film. The belt 14 and heated polymericfilm are then immersed in a coagulant bath 18 for sufficient time toeffect phase separation of the polymer solution and to form amicroporous polymeric membrane. The belt 14 is carried over one or morerollers 20 in the bath 18. The belt 14 supporting the microporousmembrane then is wound onto wind up drum 22.

[0043] As described above different arrangements could be made to carryout other embodiments of the present invention. For example, one coulduse an alternative heat source such as infrared heaters. Alternatively,the thermal source can be located on the side of the solution oppositeof the belt 14. In another embodiment a series of thermal sources can beused to prolong the thermal assistance transfer. Alternatively, one canapply a positive cooling to the side of the solution opposite the heatsource to create a greater gradient in the structure. One can also applytwo thermal sources, one to each side. They may be of the sametemperature or different, applied simultaneously or sequentially andthey can be applied for the same or different duration depending uponthe structure one wishes to make.

[0044] If desired, one can form composite membranes, i.e. membranesformed of two or more layers of membranes, in which one or more of thelayers of the composite membrane have been formed by the method of thisinvention. Typically a first support layer is formed such as anothermicroporous structure, which may be microporous structure of the presentinvention or any other microporous structure or it may be a non-woven orwoven sheet, such as TYPAR® or TYVEK® sheet materials available from E.I. DuPont de Nemours of Wilmington, Delaware or glass fiber or plasticfiber mats. The second layer is cast on to this preformed layer to formthe composite structure. One embodiment of such a structure would beform a symmetrical membrane on top of an asymmetrical membrane of thepresent invention. The symmetrical membrane may be formed according tothe present invention (which is preferred) or it may be formed by anyother known process. Alternatively, one can form a first asymmetricalmembrane according to the present invention of a specific porosity andasymmetry and then a form a second layer of the same or preferably,different asymmetry and pore size. Likewise if desired compositestructures of two or more symmetrical structures can also be formed.

[0045] Additionally, it has been found that the percentage of pore spaceon the tight side of an asymmetrical membrane (the side having thesmaller pores) is significantly greater than that which can be achievedwith any of the prior art. Typically, the methods of the prior artresulted in a surface that was ‘skinned”. By skinned, it was meant thatthe amount of open pore space on the surface is relatively small ascompared to the entire surface area. When view via a scanning electronmicroscope, one sees a surface having small open pores extending intothe structure and these pores are surrounded by large areas of solidpolymer structure. The surface is unlike that obtained with a symmetricmicroporous structure, where the surface is as open as thecrosssectional thickness. For example, the methods of the prior artdiscussed above have produced asymmetric membranes having a tight poresurface, typically having an open pore space % of from about 1 to about5% of the entire surface area. Few of the prior art methods may becapable of producing tight surfaces with a 5-10% pore space. This isunacceptable as it limits flow and reduces the flux that is capable ofbeing achieved in the membrane.

[0046] It has been discovered that the present invention is capable ofproducing asymmetric membranes with significantly higher percentages oftight side porosity than is achieved with the prior art. Asymmetricmembranes of the present invention are capable of of open pore spacepercentages of greater than 10%, typically from about 10 to about 20%and in some instances even greater than about 20%. This allows ofgreater flow and flux to be achieved than is possible with the membranesof the prior art.

[0047] The following examples illustrate the present invention and arenot intended to limit the same. A practitioner of ordinary skill in theart of developing and producing porous polymer structures, particularlyporous membranes, will be able to discern the advantages of the presentinvention. It is not the intent of the discussion of the presentinvention to exhaustively present all combinations, substitutions ormodifications that are possible, but to present representative methodsfor the edification of the skilled practitioner. Representative exampleshave been given to demonstrate reduction to practice and are not to betaken as limiting the scope of the present invention. The inventor seeksto cover the broadest aspects of the invention in the broadest mannerknown at the time the claims were made.

EXAMPLE I

[0048] The initial experiment showed that by a short exposure of a castfilm, before phase separation but after formation, to heat, the bubblepoint of a membrane can be changed drastically.

[0049] A 20 w % PVDF solution is made with N-methylpyrrolidone. Thisfilm is cast on a polyester sheet and subsequently placed on a hot stagefor different times. This heat treated film is then immersed into amethanol bath for 2 minutes and washed with water. Finally the membranesare air dried under restraint. TABLE 1 Temperature IPA peak bubble pointTreatment time (Centigrade) (psi) 10 sec 50 12 30 sec 50 20 2 × 2 sec 5010 0 (no heat treatment) — 54-50 psi

[0050] Surprisingly, the bubble point of the membranes was clearlychanged in a relatively small time frame, see Table 1.

EXAMPLE II

[0051] By using the thermal assisted casting process, membranes withimproved fluxes can be made if the exposure time is limited. To obtainbetter reproducible results, a heated rod was installed on a continuouscasting machine. The support belt was conveyed over this rod duringcasting. An air gap between the heating rod and the coagulation bath wasplaced to allow the film to cool down to reduce flammability issues inthis particular case.

[0052] A 20 w % PVDF solution was made in N-methylpyrrolidone at roomtemperature. A thin film of polymer solution was cast continuously ontoa polyester belt. This cast film was exposed for various times to aheated rod at a controlled temperature. Table 2 displays the time andtemperatures of the heat exposure. This heat treated film was quenchedinto methanol at room temperature and extracted in water, then air driedunder restraint. TABLE 2 Thickness corrected air Temperature Exposuretime flux (Centigrade) (seconds) Bubble Point (scfm/psi * um) 45 1 248.5 45 2 24 9 45 3 24 9.8   47.5 1 24 8.3   47.5 2 20 11.2   47.5 3 2011.3 50 2 14 6 Durapore ® 0.45 — 15 5.5 Durapore ® 0.22 — 27 3.5

[0053] Thickness corrected air flux (air flux times thickness) forsimilar bubble points for asymmetric membranes made by the presentprocess was observed to be twice as much as for symmetric, commerciallyavailable membranes known as DURAPORE® membranes available fromMillipore Corporation of Bedford, Mass. These thickness corrected fluxincreases are similar if not higher as the membranes formed by humid airexposure of U.S. Pat. No. 5,834,107. However, no large voids are formedto achieve this flux increase, indicative of a larger asymmetry withingthe membrane.

[0054] It is clear from Table 2 and FIG. 2 that is a graphicalrepresentation of the data that the membranes formed by this process canexhibit a higher flux compared to symmetric membranes.

EXAMPLE III

[0055] Asymmetry in PVDF Membranes: SEM Confirmation

[0056] A PVDF solution was made of 20 w % PVDF in NMP. Membranes weremachine cast by exposing a cast film (6 mil gap) to a heated rod for 2seconds. Two different types of structures were formed using samelacquer and coagulation procedure: a symmetric, unskinned PVDF membranewith low bubble point (high temperatures) and an asymmetric membrane (atintermediate temperatures). An image of such an asymmetric structure ofthe present invention is shown in cross section in FIG. 3. FIG. 4 showsthe top surface of the membrane of FIG. 3 and FIG. 5 shows the bottomsurface of the membrane of FIG. 3. TABLE 3 Peak Bubble Point Temperature(C.) (psi) 43 77 46 13

[0057] Unlike what is usually seen with basically symmetric membranes,the bath side of the membrane has a more tight and irregular structurecompared to the belt side.

[0058] In all of the examples no macrovoids or filamentous webs wereformed, thereby providing a more efficientand effective filter thanthose formed by the processes of the prior art.

I claim:
 1. A process for making a macrovoid-free microporous polymer structure having pores of an average size between about 0.02 and 10 micrometers which comprises: a) preparing a homogeneous solution consisting of at least one polymer in a solvent system consisting of at least one component that is a solvent for said polymer composition, and containing zero, one or more components which are non-solvents for said polymer composition, b) forming the polymer solution into a desired shape, c) subjecting the polymer solution of said desired shape to a thermal assist to attain a predetermined temperature profile in said desired shape, wherein the thermal assist is effected during or after forming said desired shape, d) effecting phase separation of said polymer solution and, e) recovering the microporous polymer structure.
 2. A process according to claim 1 further comprising a step (f) extracting the remaining components of the solvent system from the recovered microporous structure.
 3. A process according to claim 1 further comprising a step (g) drying the recovered microporous polymer structure.
 4. A process according to claim 1 wherein the shape of the porous polymeric structure is a thin flat sheet.
 5. A process according to claim 1 wherein the shape of the porous polymeric structure is a tube.
 6. A process according to claim 1 wherein the shape of the porous polymeric structure is a hollow fiber.
 7. A process according to claim 1 wherein the shape of the porous polymeric structure has a form of a sphere, a block or a cylinder.
 8. A process according to claim 1 wherein the porous polymeric structure is a membrane.
 9. A process according to claim 1 wherein the porous polymeric structure is a membrane and the membrane is supported on a substrate, said substrate being an integral part of the resulting membrane.
 10. A process according to claim 1 wherein the thermal assist of (c) is selected from the group consisting of heating, cooling and combinations thereof.
 11. A process according to claim 1 wherein the thermal assist of (c) is by heating.
 12. A process according to claim 1 wherein the thermal assist of (c) is by heating that is applied to one side of the structure.
 13. A process according to claim 1 wherein the thermal assist of (c) is by heating that is applied to both sides of the structure.
 14. A process according to claim 1 wherein the thermal assist of (c) is by a combination of heating and cooling.
 15. A process according to claim 1 in which the polymer is polyvinylidene fluoride.
 16. The process of claim 1 wherein the thermal assist of (c) is by and is effected to produce a thermal gradient profile through at least one portion of the thickness of the polymer solution.
 17. The process of claim 1 wherein the thermal assist of (c) is effected to produce a thermal gradient profile through the thickness of the polymer solution.
 18. The process of claim 1 wherein the thermal assist of (c) is effected to produce a uniform temperature profile through the thickness of the polymeric solution.
 19. A microporous polymeric structure made by a process according to claim
 1. 20. A microporous polymeric structure made by a process according to claim 1 wherein the structure is in the form of a membrane.
 21. A microporous polymeric structure made by a process according to claim 1 wherein the structure is in the form of a membrane and the membrane is asymmetric..
 22. A microporous polymeric structure made by a process according to claim 1 wherein the structure is in the form of a membrane and is asymmetric and is substantially free of macrovoids and filamentous webs.
 23. A microporous polymeric structure made by a process according to claim 1 wherein the structure is in the form of a membrane and is asymmetric, is substantially free of macrovoids and filamentous webs and has a surface porosity on the tight side of greater than about 10%.
 24. A microporous polymeric structure made by a process according to claim 1 wherein the structure is in the form of a membrane and is asymmetric, is substantially free of macrovoids and filamentous webs and has a surface porosity on the tight side of from about 10% to about 20%.
 25. A microporous polymeric structure made by a process according to claim 1 wherein the structure is in the form of a membrane and is asymmetric and has a surface porosity on the tight side of greater than about 10%.
 26. A microporous polymeric structure having a pore size from about 0.02 and 10 micrometers which comprises: a) preparing a homogeneous solution consisting of at least one polymer in a solvent system consisting of at least one component that is a solvent for said polymer composition, and containing zero, one or more components which are non-solvents for said polymer composition, b) forming the polymer solution into a desired shape, c) subjecting the polymer solution of said desired shape to a thermal assist to attain a predetermined temperature profile in said desired shape, wherein the thermal assist is effected during or after forming said desired shape, d) effecting phase separation of said polymer solution and, e) recovering the microporous polymer structure.
 27. A microporous polymeric structure having a pore size from about 0.02 to about 10 micrometers comprising an asymmetrical gradient in porosity from a first major surface to a second major surface and wherein the first and second major surfaces are skinless.
 28. The structure of claim 27 wherein at least the first major surface has a porosity of at least 10%.
 29. The structure of claim 27 wherein at least the first major surface has a porosity of from about 10% to about 20%.
 30. The structure of claim 27 wherein at least the first major surface has a porosity of from about 10% to about 20% and wherein the structure is in the form of a membrane and is substantially free of macrovoids and filamentous webs.
 31. The structure of claim 27 wherein the polymeric structure is formed of a material selected from the group consisting of polyvinylidene fluoride(PVDF), PVDF blends and PVDF copolymers. 